1. Field of the Invention
This invention relates to methods of creating scaffolds for artificial tissues and scaffolds and tissues made by these methods, specifically, the invention relates to methods utilizing depositing scaffolding material into a foamy substance to create soft tissues in a biologically friendly environment.
2. Description of Related Art
Tissue engineering is a rapidly developing field. The complexity of biological structures such as natural tissue have resulted in researchers exploring the techniques of rapid prototyping and other layered manufacturing techniques to create tissue constructs. Traditional methods of manufacturing are being adapted towards working within a biologically-friendly environment. This has led to the development of the new field of computer-aided tissue engineering (CATE). Solid freeform fabrication techniques (SFF) have been applied to create three dimensional shapes (3D shapes). SFF is a designation for a group of layered manufacturing techniques or processes that produce three dimensional shapes from additive formation steps. SFF, also known as Rapid Prototyping (RP), does not implement any part-specific tooling. A three dimensional part is produced from a 3D representation devised with the aid of a computer aided modeling program (CAD). This computer representation is a layer-by-layer slicing of a desired shape into consecutive two dimensional layers, which can then be fed to the control equipment to fabricate the desired part. SFF entails many different approaches to the method of fabrication. Stereolithography (SL), selective laser sintering (SLS), laminated object manufacturing (LOM), and fused deposition modelling (FDM) are examples of commercial processes applying SFF techniques.
Making scaffolds by layered manufacturing techniques requires the use of supporting structures such as, for example, sturdy surfaces or molds made of wax or polymeric foams. U.S. Pat. No. 5,503,785 to Crump, et al. details a method of creating breakaway supports for rapidly prototyped parts using a release coating. U.S. Pat. No. 6,797,351 to Kulkarni, et al. also describes methods for creating breakaway supports utilizing stereolithography. U.S. Pat. No. 5,053,090 to Beaman, et al. describes selective laser sintering where layers of powder are sintered with a laser. The unsintered powder acts as a support material and is later removed. U.S. Pat. No. 5,807,437 to Sachs, et al. uses a binder and powder system with the powder acting as a support material. U.S. Pat. No. 6,066,285 to Kumar, et al. describes using electrophotographic powder and a different support powder to build parts layer by layer. U.S. Pat. No. 6,030,199 to Tseng, et al. uses a method of depositing wax or other support materials during construction. After the part is completed, the support material is removed.
U.S. Pat. No. 6,790,403 to Priedeman, Jr., et al. describes an alkali-soluble support material that can be dissolved for a final product. U.S. Pat. No. 5,824,250 to Whalen et al. describes gel cast molding to create ceramic parts with dissolvable support materials. U.S. Pat. No. 6,375,880 to Cooper, et al. and U.S. Pat. No. 5,260,009 to Penn also describe the use of dissolvable support materials.
U.S. Pat. No. 5,697,043 to Baskaran, et al. describes a slurry made from a powder suspension in a gelling polysaccharide. A part layer is gelled and hardened, additional layers are added and hardened to eventually form the final part.
Many layered manufacturing techniques such as, for example, selective laser sintering, 3-D printing, fused deposition method, and stereolithography do not use foam as a support material. Much of the parts created using solid freeform fabrication and rapid prototyping techniques are too heavy to be supported by weak foams. Direct printing into a denser foam is difficult due to resistance and disturbs constructed layers. U.S. Pat. No. 6,021,358 to Sachs mentions the use of a foam, among other materials, as a removable filler material for rapid prototyping combined with subtractive processes.
Foams are used in a lost-foam technique for filling a mold. In the lost-foam technique, the foam is used as a temporary mold. Material is poured into the mold, destroying the foam and taking the shape of the mold. However, the method is just a variation of the lost-wax method of casting. U.S. Pat. No. 6,609,043 to Zoia, et al., describes using rapid prototyping methods to create molds filled by a foam.
U.S. Pat. Nos. 5,738,817 and 5,900,207 to Danforth, et al. utilize a dense foam base as a foundation substrate in a fused deposition method. However, the described foam material is inflexible and dense and has to be broken off or dissolved after the completion of the building process.
Other layered manufacturing methods also use sturdy foams as sheets or molds (see U.S. Pat. Nos. 5,514,232, 5,879,489, and 6,575,218 to Burns et al., U.S. Pat. No. 5,663,883 to Thomas et al., U.S. Pat. No. 5,997,681 to Kinzie, U.S. Pat. No. 6,119,567 to Schindler et al., and U.S. Pat. No. 5,876,550 to Feygin et al.).
Generally, the above techniques are not adapted for creating biologically active materials. These manufacturing techniques are geared up for creating “hard” parts found in inanimate objects, rather than “soft” or “wet” parts, that are found in biologically active systems. The use of light and pliable foams has not been explored by the current methods of rapid prototyping.
For biological scaffolds, the known applications of foams are limited to the creation of porous scaffold materials. U.S. Pat. No. 6,319,712 to Meenan et al. utilizes foams or other porous materials for artificial cartilage surfaces. U.S. Pat. No. 6,283,997 to Garg et al. describes using stereolithography to create porous ceramic structures for orthopedic implants. U.S. Pat. Nos. 6,306,424, 6,333,029, 6,365,149, and 6,534,084 to Vyakarnam et al. and U.S. Pat. No. 6,337,198 to Levene et al. disclose porous foam composites for tissue engineering. U.S. Pat. No. 6,548,569 to Williams, et al. describes using foams in medical devices. U.S. Pat. No. 6,432,435 to Timmons, et al. discloses creating keratin-based films, foam scaffolds, and sheets. Thus, in these patents, foams are used as a structural component such as, for example, a sheet of metal or an aluminum rod or a ceramic plate.
Layered manufacturing techniques have gained increased interest in the field of tissue engineering due to their ability to create complex parts and various geometries. Some of these industrial methods have been modified to be performed in a liquid and sterile environment to accommodate working with biological factors and cells.
Reischmann and Weiss et al. described a method for building bone tissue scaffolds using laminated sheets of material and stacking them together [1, 2]. Yan and Xiong et al. disclosed the concept of using layered manufacturing methods and multi-nozzle deposition extrusion and jetting processes [3, 4]. R. Landers, et al. devised a SFF method using a syringe-based system to dispense liquids, which is suitable for working with biological materials such as cells and hydrogels [5, 6, 7]. Calvert et al. devised a syringe-based system for the extrusion of hybrid polymer materials embedded with glass using layered SFF manufacturing [8]. Vozzi et al. devised a microsyringe deposition system [9, 10]. Ang et al. created a single-nozzle automated extrusion system that can utilize basic STL files [11]. U.S. Pat. Nos. 6,139,574 and 6,176,874 to Vacanti et al. disclose vascularized tissue regeneration matrices formed by solid free form fabrication techniques. U.S. Pat. No. 6,143,293 to Weiss, et al. discloses assembled scaffolds for three dimensional cell culturing and tissue generation. U.S. Pat. Nos. 6,027,744 and 6,171,610 to Vacanti et al. describe guided development and support of hydrogel-cell compositions. U.S. Pat. No. 6,454,811 to Sherwood et al. discloses composites for tissue regeneration and methods of manufacture thereof. U.S. Pat. No. 6,547,994 to Monkhouse et al. describes a process for rapid prototyping and manufacturing of primarily drug delivery systems with multiple gradients, mostly involving the three dimensional printing (3DP) technique. U.S. Pat. No. 6,623,687 to Gervasi et al. describes a process for making three-dimensional objects by constructing an interlaced lattice construct using SFF to create a functional gradient material. U.S. Pat. No. 6,183,515 to Barlow et al. utilizes selective laser sintering to create calcium phosphate bone implants.
The adaptation of these techniques for biological purposes has many obstacles. Many of the methods, such as laser sintering, stereolithography, fused deposition, and 3-D printing, create parts under operating conditions that are environmentally hostile to cell viability. These methods use high temperatures, powders, chemicals, and so forth, that does not allow cells to be introduced into the part during manufacture. These methods are only suitable for creating “hard” scaffolds that can be cleaned and processed with cells being introduced at a later time.
However, there is also a need for creating “soft” scaffolds, such as hydrogel-based scaffolds, that can sustain viable cells during manufacture. The creation of a syringe based system within a liquid environment was the next step to try to solve this problem. This technique allows printing into liquids or use low temperature to freeze the liquid to act as a support material during construction of a scaffold [12]. The liquid acts as a crosslinker to polymerize the deposited solution.
The disadvantage of the method of printing into a liquid solution is that the density of the deposited solution is very similar to the density of the liquid it is being deposited into, so that the deposited solution can be easily disturbed and can float or drift. This problem can be alleviated to some extent if the liquid level is increased in a layer-by-layer fashion and is properly regulated. However, the height of the liquid level will vary depending upon the height of the layer being constructed. Thus, slight inaccuracies in calculations multiply by each additional layer. Also, if there is a trapped air, or if the scaffold is less dense than the liquid, the buoyancy can be disruptive, and results in the part having a tendency to float.
Filaments and struts may also tend to float during the manufacturing process so that the resulting part would have features that are not as sharp or well-defined as desired. Many of the known techniques print into a crosslinking solution to create the final scaffold. Differences in density between the scaffold and the liquid solution can create problems as described above. The liquid itself does not provide much stability to the structure. In addition, the liquid may transmit forces and vibrations from the mechanical apparatus that may reduce the precision of the device. Further, there may also be diffusion of biological and chemical factors or components during the manufacturing process, especially for the construction of large tissue engineered constructs.
“Soft” parts need to have supporting structures in order to ensure stability during their manufacture. Arches and bridge-like features need support against gravity, even in a liquid environment of a similar density. To address these needs, the liquid solution could be made denser, but this would cause problems with buoyancy and viscosity. In addition, the moving parts such as a print head or a nozzle will impart forces to the previously deposited layer while traveling above it through the viscous fluid.
In current rapid prototyping and solid freeform fabrication techniques, foam is not generally used as a supporting material but as a building material and is generally dense or solidified to create “hard” parts. The known techniques are not as well developed for creating “soft” parts that are common in biological components such as soft tissue.
For biologically active scaffolds, foam is also used as a structural component due to its porous architecture. Current manufacturing techniques are a modification of industrial techniques developed for manufacturing of “hard” parts, and are not well-adapted for biological conditions due to harsh manufacturing conditions such as high temperatures, harmful chemicals, and other environmental conditions.
Newer techniques are designed to create “soft” tissue components within a liquid environment that is much more conducive to cell growth and survival. However, these methods have limitations in the manufacturing process and do not result in creating well-defined reproducible parts. Despite the foregoing developments, there is a need in the art for improved methods of making scaffolds or parts suitable for accommodation and sustaining of biologically active substances, wherein these scaffolds or parts are made to be more reproducible and more precise in their dimensions.
All references cited herein are incorporated herein by reference in their entireties.